The large-scale dynamics of quantum integrable systems is often dominated by ballistic modes due to the existence of stable quasiparticles. We here consider as an archetypical example for such a system the spin-$\frac{1}{2}$ XXX Heisenberg chain that features magnons and their bound states. An interesting question, which we here investigate numerically, arises with respect to the fate of ballistic modes at finite temperatures in the limit of zero magnetization $m=0$. At a finite magnetization density $m$, the spin-autocorrelation function $\mathrm{\Pi }(x,t)$ (at high temperatures) typically exhibits a trimodal behavior with left- and right-moving quasiparticle modes and a broad center peak with slower dynamics. The broadening of the fastest propagating modes exhibits a subdiffusive $t^{1/3}$ scaling at large magnetization densities $m\to \frac{1}{2}$, familiar from noninteracting models; it crosses over into a diffusive scaling $t^{1/2}$ upon decreasing the magnetization to smaller values. The behavior of the center peak appears to exhibit a crossover from transient superdiffusion to ballistic relaxation at long times. In the limit $m\to 0$, the weight carried by the propagating peaks tends to zero; the residual dynamics is carried only by the central peak; it is sub-ballistic and characterized by a dynamical exponent $z$ close to the value $\frac{3}{2}$ familiar from Kardar-Parisi-Zhang (KPZ) scaling. We confirm that, employing elaborate finite-time extrapolations, that the spatial scaling of the correlator $\mathrm{\Pi }$ is in excellent agreement with KPZ-type behavior and analyze the corresponding corrections.

We investigate the transport properties of tungsten trioxide $\left({\mathrm{WO}}_3\right)$ from first principles. The full $\mathbf{k}$ dependence of the scattering rate is included, and electron mobilities as a function of doping are calculated using Boltzmann transport theory. Our predicted room-temperature mobility is as high as $485{\mathrm{cm}}^2$ ${\mathrm{V}}^{-1}$ ${\mathrm{s}}^{-1}$ at ${10}^{20}{\mathrm{cm}}^{-3}$ doping, which makes ${\mathrm{WO}}_3$ a promising material for device applications. We elucidate why the mobility is so much higher in ${\mathrm{WO}}_3$ compared to the prototype perovskite oxide ${\mathrm{SrTiO}}_3$ based on an analysis of the electronic and vibronic structure.

Second harmonic generation (SHG) on the pyrochlore metal ${\mathrm{Cd}}_2{\mathrm{Re}}_2{\mathrm{O}}_7$ indicates the presence of three order parameters setting in below an inversion breaking transition. Here, we explore a possible structural explanation and relate it not only to the SHG data but also to neutron and x-ray diffraction, where we find that such a structural scenario can explain certain reflection extinctions observed in single crystal x-ray data. From this analysis, we suggest future experiments that could be done to resolve this matter. Finally, we comment on the Landau-violating nature of the inversion breaking transition and its relation to similar phenomena observed in improper ferroelectrics.

The bulk photovoltaic effect is an example of a nonlinear optical response that leads to a DC current that is relevant for photovoltaic applications. In this paper, we theoretically study this effect in the presence of electron-phonon interactions. Using the response function formalism, we find that the nonlinear optical response, in general, contains three operator correlation functions, one of which is not ordered in time. This latter correlator cannot be computed from equilibrium field theory. Using a semiclassical approach instead, we show that the bulk photovoltaic effect can be attributed to the dipole moment of the generated excitons. We then confirm the validity of the semiclassical result (which agrees with the noninteracting result) for nonlinear DC response from a quantum master equation approach. From this formalism we find that, in contrast to usual linear response, the scattering rate has a strong implicit effect on the nonlinear DC response. Most interestingly, the semiclassical treatment shows that the nonlinear DC response for spatially inhomogeneous excitation profiles is strongly nonlocal and must involve the aforementioned out-of-time-ordered correlators that cannot be computed by equilibrium field theory.

We present a first-principles approach for computing the phonon-limited $T_1$ spin relaxation time due to the Elliott-Yafet mechanism. Our scheme combines fully relativistic spin-flip electron-phonon interactions with an approach to compute the effective spin of band electrons in materials with inversion symmetry. We apply our method to silicon and diamond, for which we compute the temperature dependence of the spin relaxation times and analyze the contributions to spin relaxation from different phonons and valley processes. The computed spin relaxation times in silicon are in excellent agreement with experiment in the 50–300 K temperature range. In diamond, we predict intrinsic spin relaxation times of 540 $\mu \mathrm{s}$ at 77 K and 2.3 $\mu \mathrm{s}$ at 300 K. We show that the spin-flip and momentum relaxation mechanisms are governed by distinct microscopic processes. Our work enables precise predictions of spin-phonon relaxation times in a wide range of materials, providing microscopic insight into spin relaxation and guiding the development of spin-based quantum technologies.

We develop a theory of designing slit experiments in two-dimensional electron systems with the Rashba spin-orbit interaction. By investigating the spatiotemporal dynamics of electrons passing through a single slit or a double slit both analytically and numerically, we find that the interference fringes of the electron probability density attain specific spin orientations via the precession of spins around effective magnetic fields mediated by the Rashba spin-orbit interaction the directions of which are determined by the propagation path. The spin orientations of the fringes can be controlled by tuning the Rashba spin-orbit parameter, which can be achieved by applying an electric gate voltage. This phenomenon can be exploited to implement electrically tunable transmission of spin information and to generate spin-polarized currents.

We revisit charge density wave (CDW) behavior in the archetypal quasi-one-dimensional (quasi-1D) material ${\mathrm{NbSe}}_3$ by high-resolution angle-resolved photoemission spectroscopy measurements utilizing a microfocused laser with a photon energy of 6.3 eV. We present a detailed view of the electronic structure of this complex multiband system and unambiguously resolve CDW gaps at the Fermi level $\left(E_{\mathrm{F}}\right)$. By employing a tight-binding model, we argue that these gaps are the result of interband coupling between electronic states that reside predominantly on distinct 1D chains within the material. Two such localized states are found to couple to an electronic state that extends across multiple 1D chains, highlighting the importance of a higher-dimensional interaction in stabilizing the CDW ordering in this material. In addition, the temperature evolution of intrachain gaps caused by the CDW periodicities far below $E_{\mathrm{F}}$ deviate from the behavior expected for a Peierls-type mechanism driven by nesting; the upper and lower bands of the renormalized CDW dispersions maintain a fixed peak-to-peak distance while the gaps are gradually removed at higher temperatures. This points toward a gradual loss of long-range phase coherence as the dominant effect in reducing the CDW order parameter, which may correspond to the loss of coherence between the coupled chains. Furthermore, one of the gaps is observed above the bulk and surface CDW transition temperatures, implying the persistence of short-range incoherent CDW order. The influence of such higher-dimensional interactions likely plays an important role in a range of low-dimensional systems.

We calculate the Hall conductivity of a Sierpiński carpet using Kubo-Bastin formula. The quantization of Hall conductivity disappears when we increase the depth of the fractal, and the Hall conductivity is no more proportional to the Chern number. Nevertheless, these quantities behave in a similar way showing some reminiscence of a topological nature of the Hall conductivity. We also study numerically the bulk-edge correspondence and find that the edge states become less manifested when the depth of Sierpiński carpet is increased.

We employ a recent formulation for the optical properties of two-dimensional crystals from first principles [L. Matthes et al., New J. Phys. 16, 105007 (2014); L. Matthes et al., Phys. Rev. B 94, 205408 (2016)] to compute the surface susceptibility and surface conductivity of ${\mathrm{MoS}}_2$ and ${\mathrm{WSe}}_2$ monolayers [G. Jayaswal et al., Opt. Lett. 43, 703 (2018)]. As electron-hole interactions are known to be crucial for the description of the absorption spectrum of monolayer transition metal dichalcogenides, the excitonic dielectric function is computed at the Bethe-Salpeter equation level, including spin-orbit interactions. For both of these examples, excellent agreement with experimental ellipsometry measurements is obtained. Driven by the emergence of additional features in our theoretical results, we applied a second-derivative analysis in order to identify excited exciton peaks in the ellipsometric spectra.

Periodically driven non-Hermitian systems can exhibit rich topological band structure and non-Hermitian skin effect, without analogs in their static or Hermitian counterparts. In this work we investigate the exceptional band-touching points in the Floquet quasienergy bands, the topological characterization of such exceptional points and the Floquet non-Hermitian skin effect (FNHSE). Specifically, we exploit the simplicity of periodically quenched two-band systems in one dimension or two dimensions to analytically obtain the Floquet effective Hamiltonian as well as locations of the many exceptional points possessed by the Floquet bulk bands. Two different types of topological winding numbers are used to characterize the topological features. Bulk-boundary correspondence (BBC) is naturally found to break down due to FNHSE, which can be drastically different among different bulk states. Remarkably, given the simple nature of our model systems, recovering the BBC is doable in practice only for certain parameter regime where a low-order truncation of the characteristic polynomial (which determines the Floquet band structure) becomes feasible. Furthermore, irrespective of which parameter regime we work with, we find a number of intriguing aspects of Floquet topological zero modes and $\pi$ modes. For example, under the open boundary condition, zero edge modes and $\pi$ edge modes can individually coalesce and localize at two different boundaries. These anomalous edge states can also switch their accumulation boundaries when a certain system parameter is tuned. These results indicate that non-Hermitian Floquet topological phases, though more challenging to understand than their Hermitian counterparts, can be extremely rich in the presence of FNHSE.

The electric quadrupole-quadrupole $\left({\mathcal{E}}_{qq}\right)$ interaction is believed to play an important role in the broken symmetry transition from phase I to II in solid hydrogen. To evaluate this, we study structures adopted by purely classical quadrupoles using Markov chain Monte Carlo simulations on face centered cubic (fcc) and hexagonal close packed (hcp) lattices. Both undergo first-order phase transitions from rotationally ordered to disordered structures, as indicated by a discontinuity in both quadrupole interaction energy $\left({\mathcal{E}}_{qq}\right)$ and its heat capacity. Cooling fcc reliably induced a transition to the $Pa3$ structure, whereas cooling hcp gave inconsistent, frustrated, and $c/a$-ratio-dependent broken symmetry states. Analyzing the lowest-energy hcp states using simulated annealing, we found $P{6}_3/m$ and $Pca{2}_1$ structures found previously as minimum-energy structures in full electronic-structure calculations. The candidate structures for hydrogen phases III–V were not observed. This demonstrates that ${\mathcal{E}}_{qq}$ is the dominant interaction determining the symmetry breaking in phase II. The disorder transition occurs at significantly lower temperature in hcp than fcc, showing that the ${\mathcal{E}}_{qq}$ cannot be responsible for hydrogen phase II being based on hcp.

High perpendicular magnetic anisotropy (PMA), a property needed for nanoscale spintronic applications, is rare in oxide conductors. We report the observation of a PMA up to 0.23 MJ/${\mathrm{m}}^3$ in modestly strained (–0.3%) epitaxial ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ films which are room-temperature ferrimagnetic conductors. Spin-lattice coupling manifested as magnetoelastic effect was found as the origin of the PMA. The in-plane $x^2\text{−}{y}^2$ states of Co on tetrahedral sites play crucial role in the magnetic anisotropy and spin-lattice coupling with an energy scale of 1 meV/f.u. The elucidation of the microscopic origin paves a way for engineering oxide conductors for PMA using metal/oxygen hybridizations.

A low-temperature magnetic resonance study of the quasi-two-dimensional antiferromagnet $\mathrm{Cu}\left(\mathrm{en}\right){\left({\mathrm{H}}_2\mathrm{O}\right)}_2{\mathrm{SO}}_4$ (en = ${\mathrm{C}}_2{\mathrm{H}}_8{\mathrm{N}}_2$) was performed down to 0.45 K. This compound orders antiferromagnetically at 0.9 K. The analysis of the resonance data within the hydrodynamic approach allowed us to identify anisotropy axes and to estimate the anisotropy parameters for the antiferromagnetic phase. Dipolar spin-spin coupling turns out to be the main contribution to the anisotropy of the antiferromagnetic phase. The splitting of the resonance modes and its nonmonotonous dependence on the applied frequency were observed below 0.6 K in all three field orientations. Several models are discussed to explain the origin of the nontrivial splitting, and the existence of inequivalent magnetic subsystems in $\mathrm{Cu}\left(\mathrm{en}\right){\left({\mathrm{H}}_2\mathrm{O}\right)}_2{\mathrm{SO}}_4$ is chosen as the most probable source.

Atomic $4f$ states have been found to be essential players in the physical behavior of lanthanide compounds, at the Fermi level $E_F$ as in the proposed topological Kondo insulator ${\mathrm{SmB}}_6$, or further away as in the magnetic superconductor system $R{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ ($R$ = rare-earth ion) and in ${\mathrm{Y}}_{1-x}{\mathrm{Pr}}_x{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$, where the $4f$ shell of Pr has a devastating effect on superconductivity. In hole-doped $R{\mathrm{NiO}}_2$, the $R$ = Nd member is found to be superconducting while $R$ = La is not, in spite of the calculated electronic structures being nearly identical. We report first-principles results that indicate that the Nd $4f$ moment affects states at $E_F$ in infinite-layer ${\mathrm{NdNiO}}_2$, an effect that will not occur for ${\mathrm{LaNiO}}_2$. Treating 20% hole doping in the virtual crystal approach indicates that 0.15 holes empty the $\mathrm{\Gamma }$-centered Nd-derived electron pocket while leaving the other electron pocket unchanged; hence Ni only absorbs 0.05 holes; the La counterpart would behave similarly. However, coupling of $4f$ states to the electron pockets at $E_F$ arises through the Nd intra-atomic $4f\text{−}5d$ exchange coupling $K\approx 0.5\mathrm{eV}$ and is ferromagnetic (FM), i.e., anti-Kondo, in sign. This interaction causes spin-disorder broadening of the electron pockets and should be included in models of the normal and superconducting states of ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$. The Ni moments differ by $0.2{\mu }_B$ for FM and antiferromagnetic alignment (the latter are larger), reflecting some itineracy and indicating that Heisenberg coupling of the moments may not provide a quantitative modeling of Ni-Ni exchange coupling.

The plasticity transition, at the yield strength of a crystal, typically signifies the tendency of dislocation defects towards relatively unrestricted motion. An isolated dislocation moves in the slip plane with velocity proportional to the shear stress, while dislocation ensembles move towards satisfying emergent collective elastoplastic modes through the long-range interactions. Collective dislocation motions are discussed in terms of the elusively defined back stress. In this paper, we present a stochastic continuum model that is based on a two-dimensional continuum dislocation dynamics theory that clarifies the role of back stress and demonstrates precise agreement with the collective behavior of its discrete counterpart as a function of applied load and with only three essential free parameters. The main ingredients of the continuum theory are the evolution equations of statistically stored and geometrically necessary dislocation densities, which are driven by the long-range internal stress; a stochastic yield stress; and, finally, two local “diffusion”-like terms. The agreement is shown primarily in terms of the patterning characteristics that include the formation of dipolar dislocation walls.

Perpendicular magnetic anisotropy (PMA) in multilayer thin films promotes the development of spintronic and magnonic research fields. The origin of the PMA deserves a thorough investigation since it contains rich physics related to multiple factors in the heterostructure system. Here, we study the origin of PMA in $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) thin films sandwiched by $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Co}{\mathrm{O}}_{3-\delta }$ (LSCO) layers grown on $\mathrm{SrTi}{\mathrm{O}}_3$ substrate, using aberration-corrected scanning transmission electron microscopy. Our findings indicate that the lattice and electronic structures of LSMO can be modified through the inhomogeneous strains induced by nanodomains in LSCO layers. Moreover, we infer that the lattice distortion in LSCO at low temperatures is a driving force to reorient the spin moment from in-plane direction to out-of-plane direction in LSMO, which could be the nature of the PMA formation. The symmetry breaking in the LSMO layer induced by the domains from LSCO enhances magnetic anisotropy energy in LSMO.

We report measurements of the superconducting properties of isolated Nb nanoislands (600–2500 nm diameters) and explain their unusual behavior in terms of rare-region onset effects, predicted for random metal-superconductor granular systems [B. Spivak, P. Oreto, and S. A. Kivelson, Phys. Rev. B 77, 214523 (2008)]. We find that the island $T_c$ is strongly suppressed even at large island diameters, exceeding 1 μm. This behavior is unexpected given that conventional theories of superconductivity in small grains predict suppression of $T_c$ only at a length scale that is two orders of magnitude smaller. In addition, we observe large island-to-island variations in $T_c$ for nominally identical islands. These two experimental observations, coupled with direct measurement of grain distribution using transmission electron microscopy, conductive atomic force microscopy, and computer simulations, provide evidence for our picture in which the onset of superconductivity on an island coincides with the transition temperature of its largest constituent grain, and then spreads to other grains due to proximity coupling.

Despite a large number of nonlocal kinetic energy density functionals (KEDFs) available for large-scale calculations, most of those nonlocal KEDFs designed for the extended systems can not be directly applied to isolated systems. In this paper we propose a generalized scheme to construct nonlocal KEDFs via the local density approximation kernels and construct a family of KEDFs for simulations of isolated systems. We have implemented these KEDFs into our developed atlas software for numerical calculations of isolated systems within orbital-free density-functional theory. The performance of KEDFs has been demonstrated by several clusters encompassing Mg, Si, and GaAs. The results show that our constructed KEDFs can achieve high numerical accuracy and stability for random clusters, therefore, making orbital-free density-functional theory accessible for practical simulations of isolated systems.